Here is the Department of Defense directly assisting local law enforcement:

DOD is able— through its Joint Task Force-North—to support approximately 80 of about 400 requests per year for law enforcement assistance. These funds have been used for activities in support of law enforcement such as operations, engineering support, and mobile training teams. For example, DOD was able to provide some funding for DOD engineering units that constructed roads at the border. While DOD provided the manpower and equipment, CBP provided the materials. In addition, DOD was able to provide some funding for DOD units that provided operational support (e.g., ground based mobile surveillance unit) to law enforcement missions.

Expect to see legislation or policy shifts soon to remove what the GAO reports as limitations to the DOD providing support to local law enforcement: missions must meet various criteria (that the mission have “a valid counterdrug nexus,” and “provide a training opportunity to increase combat readiness,” for example); the Guard units cannot carry loaded weapons and so much work with Border Patrol; and murkiness about what information can or should be shared.

April 19 Update: Washington Office on Latin American (WOLA)’s report Beyond the Border Buildup, released today, puts the National Guard’s role in context, alongside DOD’s Northcom, DOJ, and DHS roles. The upshot: “…security policies that were designed to combat terrorism and drug trafficking are causing a humanitarian crisis and putting migrants in increasing danger by exposing them to abuse by authorities and organized crime.”